92 GENERAL PRINCIPLES OF ZOOLOGY. 



another. In cross-section they are readily distinguished 

 as the stratum corneum (sc) and the stratum Malpighi (sM ) 

 of the skin (Fig. 256). In the many-layered epithelia the 

 cuticle has lost its importance, and it is either an inconspic- 

 uous bounding line or is entirely absent. 



Glandular Epithelium. Glandular epithelium is dis- 

 tinguished physiologically from ordinary pavement epi- 

 thelium by the fact that it at the same time also produces 

 secretions or excretions; anatomically it is recognizable by 

 the presence of "gland cells," cells which carry on the 

 secretion and, to a greater or less extent, reveal their char- 

 acter by their structure. Characteristic glandular cells are, 

 for example, the goblet cells; here the secretion, usually 

 slime, is collected as a vitreous mass in the interior of the 

 cell, the cytoplasm, thereby compressed into a thin ex- 

 ternal wall, reminding one of that of a goblet containing 

 the nucleus at its base (Fig. 2 5 A, 26 d}. Other gland cells 

 are the granular cells, swollen bodies completely filled 

 with secretory granules (Fig. 2$A, Ko). Naturally between 

 pavement and glandular epithelium all grades of transition 

 are found. Commonly the latter name is only employed 

 when the gland cells are especially numerous, thereby 

 giving to the epithelial area the first stage of secretory 

 importance. This is especially the case with the apparatus 

 which go by the special name of " glands," among which 

 we distinguish unicellular and multicellular glands. 



Unicellular Glands. - - Unicellular and multicellular 

 glands lead to an increase of the secretory surface by in- 

 vagination. Invagination of a single cell produces the 

 unicellular glands which are chiefly found among the in- 

 vertebrated animals (Fig. 27); a gland cell here grows so 

 enormously that there is no room for it in the epithelium, 

 but it is pushed into the deeper, the subepithelial layers. 

 Here swollen up with secretion lies the cell body with its 

 nucleus; a delicate necklike projection, furnishing a pas- 

 sage for discharging, extends to the epithelial surface. 



Multicellular Glands. In the formation of multicellu- 

 lar glands, an extensive area of the glandular epithelium 



